This is the Cosmic Visibility FAQ for HeyWhatsThat.com.
Be sure to visit the main site, whose
mission is to tell you the names of the mountains you can see
when you're standing almost anywhere in the world,
and the general FAQ, which lists all the
features HeyWhatsThat has to offer.
And please sign up for our occasional email announcements.

If you're interested in the night sky, you may want to take a
look at our
eclipse site for simulations of
lunar and solar eclipses, and the
planisphere
(FAQ)
site,
where we overlay your horizon and the planets in Google Earth.

Why isn't it working with my browser?

We develop and debug this site with Firefox, so
the layout might be incorrect and some features may not work if you're using a different browser.

What's the true horizon?

Anyone can generate a sky map assuming a flat horizon. HeyWhatsThat can go a step
further. Because we've built the machinery to calculate the panorama from
anywhere in the world, we can overlay that panorama on the sky.
Here's the difference (in our Google Earth Planisphere):

Flat horizon

True horizon

Why don't I see the true horizon?

To show the true horizon, we must first calculate the panorama
for your location. Go to the
New Panorama tab on our home page
and enter the appropriate information. When the computation finishes,
hit the Night Sky view button. (Or, to view the night sky in Google Earth, hit the View in Google Earth at night button.)

Why don't I see any planets?

We currently store position data for the Sun, Moon and planets only for 2007 through 2015.
So if you specify a time eariler or later than that you won't see any solar system objects.

How accurate are planetary positions?

The horizon line does not include the effects of astronomical refraction,
which raises stars along the horizon by about one half of a degree, and should therefore
depress the horizon line by that amount.
(It does however include the much smaller effects of terrestrial refraction on the altitude of the terrain that forms the horizon,
as described in the technical FAQ.)

We currently store position data for the Sun, Moon and planets only for 2007 through 2015.
If you specify a time earlier or later than that, you won't see any solar system objects.
We store those positions -- right ascensions and declinations computed by NASA JPL's HORIZONS System --
at one minute intervals for the Moon and ten minute intervals for all other objects
and interpolate linearly within those intervals.

We compute the topocentric position for the Moon and Earth shadow, but use
geocentric positions for all the other objects (i.e. we place the Moon
in its proper place in the sky based on the observer's location on the surface of the Earth, but everything else
is still shown where it would be if you were standing at the center of the Earth, because it makes so little difference).

There is no attempt to incorporate the actual axial tilt of the planets' rotations.

The Horizons system can provide planet position data for dates between 3000 BC and 3000 AD;
please get in touch if you're interested in something earlier or later than the next two years.

How accurate are the shadows and masks?

To draw the visibility and shadow masks, we've queried NASA JPL's HORIZONS System for
a variety of quantities.
For example, to draw the mask representing the portion of the Moon visible from the Earth,
we grab the lat/lon on the Moon that's in direct line with the Earth at a given moment.
Then the visible portion of the Moon is roughly the hemisphere centered at that point.
Similarly, for the mask representing the illuminated portion of the Moon, we grab
the lat/lon on the Moon in direct line with the Sun. We've downloaded these values
for 2007 through 2015 at one minute intervals.

These visualizations do not handle eclipses.

We don't include the effect of diurnal libration, which
means we don't take into account that people on opposite sides
of the Earth see slightly different Moons (one can see a bit more
around the left edge of the Moon, the other the right).

The Sun illuminates a full hemisphere of the Moon.

The Sun illuminates a bit more than a full hemisphere of the Earth.
We assume that atmospheric refraction at the horizon is 34' and the apparent radius of the Sun is 16',
so the Sun doesn't disappear until its center is 50' below the horizon. Then geometry
tells us the angular radius of the lit portion is not 90° but 90&deg 50'.

From the Earth you can see a little less (acos moon_radius/distance_to_moon = acos 1737000/389000000 = 89.744°)
than a full hemisphere of the Moon.

Are the directions true or magnetic?

Here we use all true bearings, not magnetic, so
that the grid line marked N will pass near the North Star.
On the main site we allow you
to switch between true and magnetic bearings, so one way to determine
the difference between true and magnetic north at your location would be
to request a New panorama on the main site.
And then you'll also get your true horizon.

Most of the images were found by browsing
the NASA Planetary Photojournal,
Wikipedia,
and the Wikimedia Commons.
We converted to PNG, cropped the planets and made the backgrounds transparent, reduced sizes, and reduced the brightness on the Pluto original.
For Moon phases we reduce the brightness of a particular range of lunar longitudes on the original full Moon image.

For the Earth's shadow,
we compute the radius of the Earth's shadow's umbra and penumbra
at the distance to the Moon, and
draw the penumbra with a 50% gray, 50% transparent PNG and the
umbra with a black, 50% transparent PNG.
In the computation we use the radius of the Earth
at latitude 45° and apply Danjon's empirical rule of increasing the radius
by 1/85 to account for the Earth's atmosphere. Our results come pretty close to the
NASA Eclipse Web Site
Lunar eclipse predictions.

Here are the sites where the images can be found and credits as they appear on those pages. Uncredited images
are from the NASA Jet Propulsion Lab, the NASA Marshall Space Flight Center, and the Wikimedia Commons.